This invention relates to breaker less ignition systems and more particularly to a breakerless ignition system including a shut off switch and grounding connections for the system.
Breakerless ignition systems have become commonplace in recent years for use with small internal combustion engines such as lawn mowers, chain saws, and the like. Such systems are generally quite compact and consist of a housing in which both the coils of the system and the elecrtronic control circuit, therefore, are potted or encapsulated. For the sake of economy and ease of fabrication, as few connections should be made to the system as are possible. The systems generally include a ferromagnetic core and a permanent magnet assembly which is rotatably mounted to move cyclically past the core and to induce voltages in the coils which are mounted on the ferromagnetic core. Additionally, the systems generally include an electronic switching device for providing control of the current in a primary coil of a transformer arrangement whereby when current is conducted through the primary coil, a high voltage will be generated in a secondary coil which is used to generate a spark in a spark gap device connected to the seconday coil.
Two types of breakerless electronic ignition switching systems have been used, namely, capacitor discharge-type systems and inductive type systems. U.S. Pat. No. 4,036,201 discloses a capacitor discharge ignition system wherein a capacitor is charged by the voltage generated across a charging coil and wherein the capacitor is periodically discharged through an electronic switching device to generate a high discharge voltage pulses in the transformer secondary coil. U.S. Pat. Nos. 3,484,677 and 4,270,509 disclose inductive types of systems for generating high discharge voltage pulses in a transformer secondary coil. In most of the above identified patented systems, the primary coil, the secondary coil and the ignition circuit components are wired together and a single common ground connection is employed. Additionally, a shut off switch is provided for the electronic circuit whereby the charging circuit is rendered inoperative when the shut off switch is closed and whereby either the charging coil or the control coil are connected to ground.
Requirements for equipment incorporating breakerless ignition circuits often require that the equipment is provided with a "dead man" cut off system whereby, when the operator releases the controls of the equipment, the ignition will be quickly cut off and the engine will, therefore, stop. However, if for any reason a common ground terminal connections has been broken in the ignition systems disclosed in the above-identified patents, it is possible that the ignition system would continue to generate ignition pulses and that teh engine would not stop if the shut off switch were closed. Such failure of the cut off mechanism results from the presence of circuit connections within the circuit module so that a complete and operative circuit still exists even though the ground connection has become disconnected.
U.S. Pat. No. 4,531,500 discloses an ignition cut off system which will not be rendered ineffective in the event of a break in the ground terminal connection. In the disclosed system, one end of the primary coil is grounded separately. Further, a separate grounding connection is provided for the electronic switching circuit. In addition, a shut off switch is provided which prevents further ignition pulses from being generated when the shut off switch is in the closed positon. However, in this system, if the found connection to the shut off switch is broken for any reason, the ignition circuit would continue to operate even with the shut off switch in the closed position and the engine could therefore not be shut off. Furthermore, in the disclosed circuit, two separate grounding connections must be made to the grounding lugs located on the transformer core. One of these connections is for the primary coil, the other connection is for the electronic ignition circuit. A separate lead must be brought out from the system for connection to a shut off switch, it is, therefore, desired to provide a simple ignition system wherein a minimum of external connections are made to the ignition wherein the primary coil of the electronic ignition system is opened by the shut off switch to deenergize the system while permitting the electronic switching device to operate as the engine coasts to a stop.
Still another electronic ignition system is disclosed in U.S. Pat. No. 4,236,494, wherein the triggering circuit for the electronic switching device has been grounded by means of a safety switch. This switch is normally in the closed position so that, when the operator releases the controls of the equipment, triggering energy will no longer by supplied to the electronic switching device whereby the engine will shut off. A disadvantage of this system is that when the triggering circuit is opened, the electronic switching device will no longer function. however, since the engine does not stop instantaneously, further high voltages will be generated by the charging circuit. To prevent excessive voltages from being developed, this system has been provided with a bleed off device to remove the excess voltages from the circuit, thus adding further cost. It is, therefore, desired to provide an electronic ignition system wherein no such bleed off devices are necessary.